1. Field of the Invention
This invention relates to a device for measuring the flow rate of gas, and more particularly to a gas flow rate measuring device suitable for use for an ultrasonic gas flow meter utilizing ultrasonic waves.
2. Description of the Prior Art
In general, an ultrasonic gas flow meter, as shown in FIG. 1, is so constructed that a pair of cylindrical probe sockets 10 are mounted on a pipe 12 through which gas is to flow, in a manner obliquely opposite to each other at a suitable angle .theta. with respect to an axis of the pipe, each having an ultrasonic transreceiver body (hereinafter referred to as "probe") 14 provided therein in a manner to slightly project into an inner surface of the pipe 12. The flow rate of gas is measured by causing the probes 14 to alternately carry out the transmission and reception of ultrasonic waves in order to measure propagation times t1 and t2 in the forward and backward directions with respect to the flow of gas, seeking the following equations (1) and (2) EQU t1=L/C+Vcos.theta. (1) EQU t2=L/C-Vcos.theta. (2),
calculating the following equation (3) from the so-obtained equations (1) and (2) EQU V=1/2cos.theta.(1/t1-1/t2) (3),
and then calculating the flow rate of gas with reference to a sectional area of the pipe, wherein
V=flow velocity (m/s), PA1 L=distance between transreceivers (m) PA1 .theta.=angle between axis of propagation of ultrasonic wave and axis of pipe (degree), and PA1 C=velocity of propagation of ultrasonic wave in still gas (m/s).
FIG. 2 shows a typical structure of an ultrasonic transreceiver or gas flow rate measuring device which has been conventionally used for an ultrasonic gas flow meter. In FIG. 2, reference numeral 10 designates a socket, 12 designates a pipe through which gas flows, and 14 designates a probe. The probe 14 is mounted in the socket 10 by fitting a flange 16 provided on a probe support member 18 in a seat member 20 through a seal ring 22 and then fixing the flange 16 in the socket 10 by means of a cap nut 24.
The gas flow rate measuring device shown in FIG. 2 is also provided with a probe coupler 26. Reference numeral 28 designates a protection tube and 30 indicates a cable which is inserted through the probe support member 18 and connected to the probe 14.
In view of the above-described construction of the conventional ultrasonic transreceiver, it will be readily understood that the removal of the probe 14 from the socket 10 for the purpose of the inspection or replacement of the probe causes gas flowing through the pipe 12 to flow out through the socket 10. Thus, the inspection or replacement of the probe 14 requires the feeding of gas through the pipe to be stopped. Alternatively, it is required that the pipe 12 be provided with a bypass for flowing gas there through during the inspection or replacement.
However, the stopping of the gas flow through the pipe during the inspection or replacement of the probe is substantially impossible, because it causes the shutdown or nonoperation of a manufacturing line.
Also, the provision of such a bypass in the pipe 12 increases the manufacturing cost. This is particularly true when the pipe has a large diameter. Further, the bypass has the additional disadvantage of disturbing the distribution of velocity of gas flowing through the pipe and generating noise irrespective of the diameter of the pipe 10, thereby deteriorating the accuracy of measurement and causing the malfunction of the gas flow rate measuring device.
Accordingly, it would be highly desirable to develop a gas flow rate measuring device which is able to carry out the inspection and replacement of a probe as desired without stopping the flow of gas through a pipe or providing the pipe with any bypass.